Digital subscriber line (DSL) technology can use the existing copper twisted pair networks used in the analog telephone network. The copper wiring is said to form communication lines or loops. At spaced ends of the communication lines are located transceivers (for example, modems) or other transmitters and receivers for respectively sending and receiving digital signals communicated along the loops.
Twisted pairs are typically bundled together in a common physical sheath, known as a binder; all twisted pairs within a bundle are said to belong to a certain binder group. Within such a binder group, these twisted pair lines are sufficiently close such that electromagnetic radiation from one pair can induce “crosstalk” interference into one or more other pairs. Therefore a signal sent along a communication line and received by a modem can comprise the transmitted signal and one or more interference signals from adjacent communication lines. In turn, these crosstalk signals forms spurious noise that interferes with intended transmissions. In general, crosstalk effects in addition to long loop lengths are the main obstacles to reaching higher data rates in such copper-based networks.
Near end crosstalk (NEXT) is caused by transmitters interfering with receivers on the same side of the bundle and is often avoided by using non-overlapping transmit and receive spectra (frequency division duplex; FDD) or disjoint time intervals (time division duplex; TDD).
Far end crosstalk (FEXT) is caused by transmitters on opposite sides of the bundle. In some cases this interference can be 10 to 20 decibel larger than the background noise and has been identified by some as the dominant source of performance degradation in DSL systems.
Telephone companies are increasingly shortening the loop using remote terminal (RT) deployments, resulting in lower signal attenuation and larger available bandwidths. Unfortunately this can cause other problems such as the “near far” effect due to the crosstalk. Common in code-division multiple access (CDMA) wireless systems, the near-far effect occurs when a user enjoying a good channel close to the receiver overpowers the received signal of a user further away having a worse channel and where both users transmit at the same power levels.
One of the shortcomings of current multi-user communication systems is power control. In typical communication systems, interference limits each user's performance. Further the power allocation of each communication line depends not only on its own loop characteristics, but also on the power allocation of all other communication lines as exemplified by the near-far effect described above. Therefore the digital communications system design should not treat each user independently, but rather consider the power allocation of all communication lines jointly.
Dynamic spectrum management (DSM) has the potential to dramatically improve the performance of digital subscriber line (DSL) networks well-beyond the current status quo, and not surprisingly, looks to be a key fixture of all future-generation DSL standards. Its focus is on relieving the problems caused by crosstalk, brought on by the close proximity of twisted copper pairs that are packed into distribution binders of 25-100 lines on their way to the exchange. While long loop lengths certainly inhibit performance, the crosstalk problem has been recognized as the critical impairment with, for example, interference contributions of 10-20 dB over the background noise.
Two competing paradigms for addressing the crosstalk phenomena are known: vectoring and spectrum balancing. Vectoring treats the DSL network as a multiple-input multiple-output (MIMO) system, where modems co-ordinate at the signal level to effectively remove crosstalk through successive decoding or preceding.
DSM techniques are not just limited to the networks of the future. Iterative water-filling (IWF) has proved to be an attractive method for optimizing power spectral densities (PSDs) of loops without centralized co-ordination.
In contrast, spectrum balancing involves a much looser level of coordination. Modems employ a low-complexity single-user encoding and decoding strategy while treating interference as noise. Early static spectrum management (SSM) efforts attempted to define static spectra of various DSL services, in an attempt to limit the crosstalk interference between DSLs that may be deployed in the same binder. The notion of DSM goes further by allowing loops to dynamically negotiate a spectrum allocation that effectively avoids crosstalk, thereby enabling significant improvements in overall network performance.
Early work in the area of DSM introduced an iterative water-filling (IWF) scheme to balance user power spectrum densities (PSDs), where each user repeatedly measured the interference received from other users, and then determined their own power allocation according to a water-filling distribution without regard for the subsequent impact on other users. This process results in a fully distributed algorithm with a reasonable computational complexity.
More recent efforts have focused on the underlying optimization problem that spectrum balancing aims to solve. Unfortunately this optimization is a difficult nonconvex problem. As such, the Optimal Spectrum Balancing (OSB) algorithm makes use of a grid-search to find the optimal power allocation to a predetermined quantization of user powers. It suffers from an exponential complexity in the number of users, and so near-optimal Iterative Spectrum Balancing (ISB) algorithms were developed that reduce complexity through a series of line-searches, avoiding the grid-search bottleneck. Both of these algorithms are centralized and are not well-suited for practical implementation.
Unfortunately, many have shown that IWF can be far from optimal. The performance gap therein lies with the damage one user's power allocation has on the performance of others: IWF does not explicitly take such information into account.
It is an object of the invention to alleviate at least in part one or more of the problems of the prior art or at least provide an alternative approach. More particularly, embodiments of the invention attempt to provide a readily useable practical system without being overly complex.